In principle, OLEDs are outstandingly suitable for the production of large-area illumination and display applications but can be found only in devices of small format at the moment due to extensive production methods. OLEDs are generally implemented in layer structures. For better understanding, FIG. 1 shows a basic structure of an OLED. Owing to the application of external voltage to a transparent indium tin oxide (ITO) anode and a thin metal cathode, the anode injects positive holes, and the cathode negative electrons. These differently charged charge carriers pass through intermediate layers, to which also hole or electron blocking layers not shown here may belong, into the emission layer. The oppositely charged charge carriers meet therein at or close to doped emitter molecules, and recombine. The emitter molecules are generally incorporated into matrices molecules or polymer matrices (in, for example, 2 to 10% by weight), the matrix materials being selected so as also to enable hole and electron transport. The recombination gives rise to excitons (=excited states), which transfer their excess energy to the respective electroluminescent compound. This electroluminescent compound can then pass into a particular electronic excited state, which is then converted very substantially and with substantial avoidance of radiationless deactivation processes to the corresponding ground state by emission of light.
With a few exceptions, the electronic excited state, which can also be formed by energy transfer from a suitable precursor exciton, is either a singlet or triplet state, consisting of three sub-states. Since the two states are generally occupied in a ratio of 1:3 on the basis of spin statistics, the result is that the emission from the singlet state, which is referred to as fluorescence, leads to maximum emission of only 25% of the excitons produced. In contrast, triplet emission, which is referred to as phosphorescence, exploits and converts all excitons and emits them as light (triplet harvesting) such that the internal quantum yield in this case can reach the value of 100%, provided that the also excited singlet state, which is above the triplet state in terms of energy, relaxes fully to the triplet state (intersystem crossing, ISC), and radiationless competing processes remain insignificant. Thus, triplet emitters, according to the current state of the art, are more efficient electroluminophores and are better suitable for ensuring a high light yield in an organic light emitting diode.
The triplet emitters suitable for triplet harvesting used are generally transition metal complexes in which the metal is selected from the third period of the transition metals. This predominantly involves very expensive noble metals such as iridium, platinum or else gold (see also H. Yersin, Top. Curr. Chem. 2004, 241, 1 and M. A. Baldo, D. F. O'Brien, M. E. Thompson, S. R. Forrest, Phys. Rev. B 1999, 60, 14422). The prime reason for this is the high spin-orbit-coupling (SOC) of noble metal central ions (SOC constants Ir(III): ≈4000 cm−1; Pt(II): ≈4500 cm−1; Au(I): ≈5100 cm-1; Ref.: S. L. Murov, J. Carmicheal, G. L. Hug, Handbook of Photochemistry, 2nd Edition, Marcel Dekker, New York 1993, p. 338 ff). Due to this quantum mechanical characteristic, the triplet-singlet transition, which is without SOC strictly forbidden for optical transitions, is allowed and an emission lifetime of a few μs, needed for OLED applications is achieved.
Economically, it would be highly advantageous if these expensive noble metals could be replaced with less expensive metals. Moreover, a large number of OLED emitter materials known to date are ecologically problematic, so that the use of less toxic materials would be desirable.